Radio Frequency IDentification (RFID) systems typically include RFID tags and RFID readers. RFID readers are also known as RFID reader/writers or RFID interrogators. RFID systems can be used in many ways for locating and identifying objects to which the tags are attached. RFID systems are particularly useful in product-related and service-related industries for tracking objects being processed, inventoried, or handled. In such cases, an RFID tag is usually attached to an individual item, or to its package.
In principle, RFID techniques entail using an RFID reader to interrogate one or more RFID tags. The reader transmitting a Radio Frequency (RF) wave performs the interrogation. The RF wave is typically electromagnetic, at least in the far field. The RF wave can also be magnetic in the near field.
A tag that senses the interrogating RF wave responds by transmitting back another RF wave. The tag generates the transmitted back RF wave either originally, or by reflecting back a portion of the interrogating RF wave in a process known as backscatter. Backscatter may take place in a number of ways.
The reflected-back RF wave may further encode data stored internally in the tag, such as a number. The response is demodulated and decoded by the reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data can denote a serial number, a price, a date, a destination, other attribute(s), any combination of attributes, and so on.
An RFID tag typically includes an antenna system, a radio section, a power management section, and frequently a logical section, a memory, or both. In earlier RFID tags, the power management section included an energy storage device, such as a battery. RFID tags with an energy storage device are known as active or semi-active tags. Advances in semiconductor technology have miniaturized the electronics so much that an RFID tag can be powered solely by the RF signal it receives. Such RFID tags do not include an energy storage device, and are called passive tags.
Harvesting sufficient power from the RF wave can be difficult since the voltage of the RF signal is in the range of approximately 200 mV, and a typical supply voltage for circuits of the RFID tag is 1 V. Available RF signal amplitude is insufficient to operate the RFID tag, therefore power rectifier circuits typically use charge-pumps to increase the output DC voltage.
Additionally, for relatively high-voltage operations, such as programming and erasing non-volatile memory in the RFID tag, a boosted voltage as high as 12 V may be needed. Complicating matters is that the RF wave received by the RFID tag is not provided constantly, and can cease to be transmitted by the RFID reader without any notice. Additionally when the RFID tag backscatters it stops harvesting energy, causing yet another interruption in voltage generation.
Thus, the operation of the passive RFID tag, converting the low-level RF waveform to a usable voltage, requires a rectifier circuit that can maximize the use of available radiation energy, and generate usable voltage quickly and efficiently.